Electronic documents have become the de facto standard for information creation, transmission and storage. An electronic document (or electronic file or electronic object) typically has content information (such as text, graphics, equations, tables, spreadsheets, pictures, sound, full motion video and other multimedia content, etc.) and formatting information which directs how the content is to be displayed. Different types of content information are more readily understood if formatted in a particular manner. Spreadsheets, for example, are more easily understood if formatted in interactive tables. Many different application programs or languages have been developed for displaying or formatting content information in specific ways. Each language, such as Microsoft Word, Adobe Acrobat, HTML, etc., defines a set of formatting rules which determines how the content information (such as a letter, article, sales information, etc.) will be displayed to a user.
Most users have an interest in protecting content information, especially when the document is not in use. The level of protection desired by users depends on the value of the content information. The more valuable the content information, the more stringent the protections desired by users. To accommodate this, many application programs include methods for protecting content. Microsoft Word, for example, allows users to protect documents by assigning a password. The Word document is stored in the clear, so an enterprising hacker could remove the password and see the document content.
The “Portable Document Format”, PDF, created by the Adobe Acrobat presents content information in a format which preserves the publisher's desired presentation appearance and it is platform independent, which makes it attractive for use over the Internet. The Acrobat software provides some document “protection.” A PDF document can be encrypted to protect its content. Encryption applies to all strings and streams in the PDF file, but not to other objects which are used to convey information about the document's structure rather than its content. Access to the “protected” PDF document is by a user password. While content is protected via encryption, PDF does not provide for a “secure” transmission, i.e., transmission where the structure and/or other information about the document is not visible. Even with partially encrypted content, however, much can be learned about a document by observing the relationship of formatted content or by comparing one version of a digital document with another. Also, the PDF does not provide for user selected encryption levels, nor for the protection of any preliminary information (which frequently reveals structure of the document) that is presented to the user in order to input the password that any user can read before opening the protected parts.
Many application programs and languages provide for insertion of comments, footnotes, and other types of annotations to an existing document. Annotations are frequently made by persons other than the original document author(s). Whether a non-author can make an annotation to another user's document depends on the particular application program and whether the non-author has been given authorization to make annotations. In a Microsoft Word document, for example, any user having access to a document can insert annotations. In Adobe Acrobat, the user's version of the Acrobat software determines whether the user can make annotations or not. If the user's version permits annotations, then in the normal sequence of events the user will save changes to the annotations causing the Acrobat software to include the new annotation and/or changes to existing annotations in a new version of the existing PDF file.
Such annotations, however, can be read by all users who have the right to open the document (whether it is a Word document or a PDF or some other document type). There is no way to limit who can read or edit the annotations a user added to the file if the new version is distributed to other users with access to a software application that shows/edits annotations. Protection of documents, for example, in which a security mechanism only provides for limiting access to those who know the password for the document cannot be considered a general solution.